docs(agents): clarify subagent progress reporting

[shunk031]

Summary

• add shared guidance for parent agents to surface delegated subagent work immediately after spawn
• require periodic polling, milestone updates, and proactive stalled-state reporting for long-running subagents
• keep the shared exact_agents guidance as the literal prefix of codex/AGENTS.md while adding the same progress-visibility section there

Testing

• GitHub Actions: Unit test

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.65%. Comparing base (a244058) to head (c96d2ef).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #439      +/-   ##
==========================================
+ Coverage   89.27%   89.65%   +0.38%     
==========================================
  Files           9        9              
  Lines         233      232       -1     
==========================================
  Hits          208      208              
+ Misses         25       24       -1     

Flag	Coverage Δ
macos-14-client	41.66% <ø> (ø)
ubuntu-latest-client	89.61% <ø> (+1.67%)	⬆️
ubuntu-latest-server	89.22% <ø> (ø)

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